This invention relates, generally, to methods for making an improved detector array and more specifically a method for making a monolithic patterned dichroic filter detector array for spectroscopic imaging by coupling a patterned dichroic filter to a linear or two dimensional detector array such as a Charge Coupled Device, and the like.
Color-dyed gels positioned in front of detector arrays are currently employed to produce spectral detectors. The gels have a number of shortcomings, however, that limit their utility. For example, they do not function over a wide temperature range and thus cannot meet strict specifications of the type that might be called for in military applications, for example. Moreover, gels absorb relatively high levels of light and thus require more power to produce a desired detection level than would a less absorptive material. Moreover, the colors of the gels are not easily controlled during their manufacture, it is difficult to formulate gels with specific spectral features, and the filtering and transmitting of colors is problematic.
Gel and absorptive technology has been used to make red green blue sensitive cameras for the detection of color. Attempts have been made, with limited success, to relate these visual color functions to the estimation of certain chemical parameters. Alternatively, non-imaging or single point measurements made with spectrometers have been widely used with great success to relate the reflection or transmission of a variety of wavelengths to the concentration or level of many industrial, medical and environmental parameters.
It has been recognized by the art that a dichroic or interference filter array would be superior to gels in numerous respects when used in conjunction with linear or two dimensional detectors such as CCDs, due to the superior optical qualities of a dichroic filter as compared to that of gels. Dichroic filters, also known as interference filters, are constructed by depositing one or more layers of metallic and dielectric films with precise thicknesses to produce filters which transmit certain wavelengths of light and reflect others. The colors of a dichroic filter can be predicted and manufactured to match spectral functions such as the CIE tristimulus curves s (e.g. the 1976 UCS standard chromaticity diagram), and such filters enable purer color filtering and transmission compared to gels due to their higher extinction ratio at wavelengths which are blocked and higher transmission at wavelengths that are passed. They are temperature stable from a range of about xe2x88x9240 degrees to 400 degrees F.; they absorb less than five percent (5%) of the light transmitted through them as they are primarily rejecting out of band wavelengths through reflection; and, for in band wavelengths, they exhibit a ninety percent (90%) transmission and thus require less power to achieve greater brightness.
The industry still uses gels, however, because it has been unable to overcome the manufacturing difficulties encountered in making an array of dichroic filters on a wafer. A typical array includes a plurality of discrete filters arranged in rows and columns on the face of the wafer. The manufacturing of dichroic filter arrays is problematic because it is difficult to manufacture a thin filter unit having sharply defined edges. If a filter unit is too thick, it absorbs too much light and thus requires high power consumption if a good image is to be produced, just like a gel. If the edges are not sharply defined, it produces low quality, hard to control color, just like a gel. These limitations exist in the manufacture of dichroic filter arrays because the art has attempted to make optical filter arrays employing etching techniques that have never been perfected. Thus, although in theory a dichroic filter array should perform better than an array of gels, in practice the thick, poorly defined dichroic filters perform just as poorly. Several large corporations, including electronic giants such as Sony, have spent millions of dollars over several years trying, without success, to develop arrays of thin, sharply edged dichroic filters on a wafer to replace the gels. However, the efforts have been futile because they are based on refinements of the optical arts. Specifically, the efforts relate to improvements in etching techniques that are designed to reduce the filter thickness and to sharpen the filter edges. More particularly, in the etching process, the filter material is deposited onto a wafer by an evaporation technique known as xe2x80x9chot process,xe2x80x9d and a protective film of copper or other suitable material is then deposited atop the filter. A photoresist layer is then deposited atop the copper; the result is a sandwich including, from the top, a layer of photoresist, a layer of protective copper, the filter material, and a wafer. Efforts are then made to etch away the photoresist and the edges of the filter material so that a square or rectangular block of filter material is left on the wafer. The copper layer immediately atop the filter material must also be etched away, but the contiguous copper must be left in place to protect the contiguous filter material when the etching is repeated to form the next block of filter material. Due to the small sizes of the filters (typically, a filter is about 20 microns in width), and since each filter must abut a contiguous filter, the task of producing an array of thin filters with sharply defined edges by conventional etching techniques is nearly impossible, as proven by the years of expensive yet unsuccessful research mentioned above.
The above problem was solved as applied to LCDs to produce color images and disclosed in U.S. Pat. No. 5,711,889, Method For Making Dichroic Filter Array, which is hereby fully incorporated into this specification.
The improvement now disclosed by this specification relates to the application of the Dichroic Filter Array shown in U.S. Pat. No. 5,711,889 to detectors thus producing the first high resolution application of dichroics in a CCD application to make imaging systems capable of color, chemical or composition related parameter detection.
This improved technology captures the analytical specificity of spectral analysis in a monolithic thin film optical filter or filters, and places these filters over discrete sensors in an imaging sensor to produce spectrally discriminated images. In addition, the filter technology has no moving parts, long life expectancy, excellent optical efficiency, and high performance.
Using Microlithographic patterned vacuum deposited thin films, where these thin films produce optical filters with transmission characteristics designed to emulate the weighted spectral response of specific chemicals, composition related parameters such as octane number in fuels or cancerous tissue in optical biopsies, or other spectral functions of interest such as UVA, TVB and UVC bands of ultraviolet radiation, photopic curves, perceived color functions, industry specific standards such as APHA water color, or any spectrally dependent function, separate filters can be produced in a pattern to measure simultaneously several chemicals or parameters. The filters are cast in patterns that overlay the detector elements in imaging detectors such that, when they are placed on a two dimensional detector in a camera, an imaging system capable of showing spatial variation in chemicals or parameters is produced.
A further advantage of the disclosed invention is the ability to produce filters for both positive and negative factors in a spectral model, or for multiple chemical species. When these different filters are deposited on adjacent pixels, they combine to form a set of pixels from which the model result can be more accurately calculated. This combination of data from adjacent pixels is accomplished typically in software such that the model results are predicted across the whole detector array, forming a single image. As mentioned earlier typical detector arrays use patterned dye gels to produce color discrimination. Patents disclosing this technique, and attempting improvements on the state of the art generally include U.S. Pat. No. 5,719,074, Hawkins, et al, Feb. 17, 1998, Method of Making a Planar Color Filter Array for CCDS From Dyed And Mordant Layers. This patent discloses an image sensor that includes an integral color filter array and a method of making such sensor. The sensor includes a semiconductor substrate having an overlying support layer with an optically planar surface, a plurality of spaced image pixels formed in the substrate; and an array of contiguous color filter elements overlying the planar surface whose top surfaces are coplanar and which have no overlap of color filter material between adjacent color filter elements. Also, U.S. Pat. No. 5,756,239, Wake, May 36, 1998, Method of Forming a Color Filter Array With Improved Resolution, discloses a method for use in forming a high resolution color filter array in which the following steps are used: coating a colored layer containing a binder, a colorant in the binder, the binder being transparent over the entire visible electromagnetic spectrum and remaining so even after extended treatment with elevated temperature and light; hardening the colored layer; providing and patterning a photoresist layer over the hardened colored layer; and treating the patterned photoresist layer so that it is selectively resistant to oxygen plasma etch. Further processing steps are used to complete the colored filter array. U.S. Pat. No. 5,954,559, Holmberg, Sep. 21, 1999, Color Filter Structure and Method of Making, discloses an improved planar color filter structure to reduce defects in the display devices incorporating the color filter structures, including active matrix displays. A color filter substrate has a thicker polyamide black matrix formed thereon and a transparent polyamide layer formed over the black matrix. The transparent layer is exposed through the black matrix and developed to remove the unexposed portions over the black matrix. The resulting surface is substantially planar and facilitates the forming of the remaining layers to form a substantially planar color filter structure. These patents disclose use of dyes and gels patterned to form the color filter. No dichroics are used.
U.S. Pat. No. 5,889,227, Hawkins, et al, Mar. 30, 1999, Planar Color Filter Array For CCDS With Embedded Color Filter Elements, discloses an image sensor and method of making such sensor. The sensor includes an integral color filter array, comprising: a semiconductor substrate having an optically planar top surface; a plurality of spaced image pixels formed in the substrate; and an array of physically contiguous color filter elements embedded in the substrate whose top and bottom surfaces are coplanar and which have no overlap of color filter layers between adjacent color filter elements. This patent discloses a planar filter element created by etching rather than the liftoff method disclosed in U.S. Pat. No. 5,711,889, and thus is difficult to manufacture.
Also, a number of patents have been issued that disclose using dichroic filters with detectors. U.S. Pat. No. 5,942,762, Hecht, Aug. 24, 1999, CCD Scanner Having Improved Specular Reflection Discrimination, discloses an optical scanner that utilizes two linear CCD detectors and a bandpass means to improve the ability of the scanner to discriminate against specular reflection. A coded symbology is illuminated by a noncoherent light source and light reflected from the coded symbology along a first path strikes the front face of the bandpass means. The bandpass means, functioning as a notch filter, transmits a select bandwidth of light while reflecting all other light onto a first CCD detector. Simultaneously, light reflected from the bar code symbol travels along a second path, at a different angle with respect to the plane of the coded symbology than the first path, is reflected from a mirror onto the back face of the bandpass means. The bandpass means transmits the select bandwidth of light onto a second CCD detector and reflects all other light. The second CCD detector has a notch filter which permits the detection of only the select bandwidth. Since specular reflection is only experienced at a single angle, with respect to the plane of the coded symbology and each CCD detector detects an image at a different angle with respect to the plane of the coded symbology, a complete image can be reconstructed by combining information obtained from both CCD detectors. But this patent does not disclose patterning on a single CCD substrate. Also, U.S. Pat. No. 5,912,451, Gurevich, et al., Jun. 15, 1999, Moving Beam And Field Of View Readers With Dichroic Filter, discloses an optical reader for reading indicia such as bar codes comprises a first and second light source for generating first and second laser beams. The respective laser sources generate light at different wavelengths and a dichroic filter is provided to allow either source to be used without parallax effects. The laser scanner beam can be used to aim the reader when carrying out field of view reading. The optical reader is further provided with a band-pass filter shaped to match the wave-front of light generated at a given location incident on the filter to reduce the band-pass bandwidth and hence the ambient noise. The shaped band-pass filter can be incorporated in the optical reader dichroic filter arrangement. This is a single bandpass filter device with no patterning or direct placement on the detector as disclosed in this application.
These prior art devices consisted mainly of patterned dye gels to produce color discrimination. This is the first high-resolution application of dichroics in a CCD application. In addition, this permits the first use of a monolithic array of discrete optical filters to emulate mathematical filters or models in an imaging system. This allows for the imaging of differences in concentration of chemicals or in levels of composition related parameters with a low cost solid state device. The filters provide performance, wavelength selectivity, longevity, and flexibility of application. The array of filters are also monolithic allowing for greater thermal stability and physical durability
This breakthrough in optical filter detector array production is made possible by uniting two separate and divergent technologies. The art of microlithography has long been employed to produce microelectronic devices, and the optical arts have long been employed to produce dichroic filter arrays. As mentioned earlier, the optical arts have failed to produce thin filters having well-defined edges, and the art of microlithography has been limited to the field of microelectronics. The present invention merges the divergent arts of microlithography and microelectronics. A xe2x80x9ccold process,xe2x80x9d well known in the art of microelectronics, is employed to deposit the filter material, in lieu of the conventional xe2x80x9chot process.xe2x80x9d Photoresist is applied to the wafer prior exposure and over development to create an undercut, thereby weakening the walls formed by the photoresist. The filter material is then deposited onto the wafer in the space created by the over development. The photoresist is then removed, thereby leaving on the wafer a thin optical filter having sharply defined edges. The thin optical filters can be designed with transmission characteristics designed to emulate the weighted spectral response of specific chemicals, or composition related parameters. Separate filters can be produced in a pattern to measure simultaneously several chemicals or parameters. The filters are cast in patterns that overlay the detector elements in imaging detectors. The resulting optical filters are then bonded to the detector or alternatively to a piece of glass that is then bonded to the detector. When placed on a two dimensional detector in a camera, an imaging system capable of showing spatial variation in chemicals or other parameters is produced.
It is therefore clear that a primary object of this invention is to advance the art of optical filter detector array manufacture. A more specific object is to advance said art by providing a method for the manufacture of monolithic patterned dichroic filters having sharply defined edges which can then be bonded to detectors to create a high performance spectral detector for spectroscopic imaging.
These and other important objects, features, and advantages of the invention will become apparent as this description proceeds. The invention accordingly comprises the features of construction, combination of elements and arrangement of parts that will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.